1. Field of the Invention
The present invention relates to a sensor which utilizes attenuated total reflection (ATR), such as a surface plasmon sensor which enables quantitative analysis of a specific material contained in a specimen by utilizing generation of surface plasmons.
2. Description of the Related Art
In metal, free electrons move collectively to produce a compressional wave called a plasma wave. When a plasma wave generated at a surface of the metal is quantized, the plasma wave is regarded as surface plasmons.
The surface plasmons can be produced by exciting a surface of a metal by an optical wave. Conventionally, various surface plasmon sensors are proposed for performing a quantitative analysis of a material contained in a specimen by utilizing the excitation by an optical wave. In particular, surface plasmon sensors which use a system called Kretschmann""s arrangement are well known (Refer to Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon sensors which use the above system basically include: a dielectric block having a form of a prism; a metal film formed on a face of the dielectric block and in contact with a specimen; a light source producing a light beam, an optical system injecting the light into the dielectric block at various incident angles which are greater than a critical angle for total reflection, and attenuated total reflection (ATR) due to a surface plasmon resonance occurs; and a light detection unit which can detect the state of the attenuated total reflection (i.e., the state of the surface plasmon resonance) by measuring the intensity of the light beam totally reflected from the above boundary.
The above various incident angles can be realized by deflecting a relatively thin light beam so that the deflected beam is incident on the boundary at desired incident angles. Alternatively, the various incident angles can be realized by letting a relatively thick light beam be incident on the boundary so that the thick light beam converges or diverges at the boundary, and therefore the converging or diverging beam contains components incident on the boundary at the various incident angles. When the relatively thin light beam is deflected, the light beam reflected at a reflection angle which varies with the deflection of the incident light beam can be detected by a small light detector which moves corresponding to the deflection of the incident light beam, or by an area sensor extending in the direction of the variation of the reflection angle. When the relatively thick light beam is incident on the boundary, the reflected light beam can be detected by an area sensor which extends in the direction of the variation of the reflection angle so that substantially all the reflected light beam can be detected.
When a light beam is incident on the metal film in the surface plasmon sensor having the above construction at a specific incident angle xcex8SP which is greater than a critical angle for total reflection, an evanescent wave is generated, where an electric field of the evanescent wave is spread in the vicinity of the metal film in the specimen. By the evanescent wave, surface plasmons are generated at the boundary between the metal film and the specimen. When the wave number of the evanescent wave equals the wave number of the surface plasmons, i.e., these wave numbers match, the evanescent wave is resonant with the surface plasmons, and the energy of the evanescent wave is transferred to the surface plasmons. Therefore, the intensity of the light totally reflected by the boundary between the dielectric block and the metal film sharply decreases. The decrease in the intensity of the light is detected as a dark line by the light detection unit.
The above resonance occurs only when the incident light beam is a p-polarized light beam. Therefore, it is necessary to arrange the surface plasmon sensor so that the light beam is incident on the boundary as a p-polarized light beam.
When the wave number of the surface plasmon is obtained from the incident angle xcex8SP at which the attenuated total reflection (ATR) occurs, the permittivity of the specimen can be obtained from the wave number of the surface plasmons. That is,                     K        SP            ⁡              (        ω        )              =                  ω        c            ⁢                                                                  ϵ                m                            ⁡                              (                ω                )                                      ⁢                          ϵ              s                                                                          ϵ                m                            ⁡                              (                ω                )                                      +                          ϵ              s                                            ,
where the wave number of the surface plasmon is denoted by KSP, the angular frequency of the surface plasmon is denoted by xcfx89, the velocity of light in vacuum is denoted by c, and permittivities of the metal and the specimen are denoted by xcex5m and xcex5s, respectively.
When the permittivity xcex5s of the specimen is obtained, the concentration of the specific material in the specimen can be obtained based on a predetermined calibration curve or the like. Therefore, properties relating to the permittivity (i.e., the refractive index) of the specimen can be obtained by detecting the incident angle xcex8SP at which the intensity of the reflected light decreases.
In addition, the leakage mode sensor is known as another sensor which is also utilizes the attenuated total reflection and similar to the surface plasmon sensor. For example, the leakage mode sensor disclosed in xe2x80x9cSpectral Researchesxe2x80x9d, Vol. 47, No. 1 (1998) pp. 21-23 and 26-27 includes: a dielectric block having a form of a prism; a cladding layer formed on a face of the dielectric block; an optical waveguide layer formed on the cladding layer and in contact with a specimen; a light source producing a light beam, an optical system which injects the light beam into the dielectric block at various incident angles so that the light beam is totally reflected at the boundary between the dielectric block and the cladding layer, and attenuated total reflection (ATR) due to excitation of a propagation mode in the optical waveguide layer can occur; and a light detection unit which can detect the state of the attenuated total reflection, i.e., the state of the excitation of the propagation mode, by measuring the intensity of the light beam totally reflected from the above boundary.
When the laser beam is incident through the dielectric block on the cladding layer in the above leakage mode sensor at a incident angle which is greater than the critical angle for total reflection, only a portion of light being incident on the cladding layer at a specific incident angle and having a specific wave number can propagate in the propagation mode in the optical waveguide layer. Therefore, when the propagation mode is excited, almost all portions of the incident light can enter the optical waveguide layer, i.e., the attenuated total reflection, in which the intensity of light totally reflected from the boundary sharply decreases, occurs. At this time, the wave number of the propagated light depends on the refractive index of the specimen placed on the optical waveguide layer. Therefore, it is possible to obtain the refractive index of the specimen and analyze other properties of the specimen relating to the refractive index.
Incidentally, in the conventional sensors utilizing the attenuated total reflection such as the surface plasmon sensors and leakage mode sensors, semiconductor laser devices are used as the light sources. However, in the conventional sensors using the semiconductor laser devices as the light sources and utilizing the attenuated total reflection, sometimes, the output of the light detection unit, which detects the state of the attenuated total reflection, suddenly varies, and resultantly the precision of measurement deteriorates.
An object of the present invention is to provide a sensor utilizing the attenuated total reflection, in which sudden variation in the output of a light detection unit is prevented so that high precision of measurement is achieved.
(1) According to the first aspect of the present invention, there is provided a sensor comprising: a dielectric block; a thin film formed on a face of the dielectric block and in contact with a specimen; a semiconductor laser unit as a light source which emits a light beam; a first optical system which injects the light beam into the dielectric block so that the light beam is incident on a boundary between the dielectric block and the thin film at a plurality of incident angles which are greater than a critical angle for total reflection; and a light detecting unit which detects a state of attenuated total reflection by measuring the intensity of the light beam totally reflected from the boundary. In the sensor, the semiconductor laser unit is driven with a driving current on which a high frequency component is superimposed.
(2) According to the second aspect of the present invention, there is provided a sensor comprising: a dielectric block; a metal film formed on a face of the dielectric block and in contact with a specimen; a semiconductor laser unit as a light source which emits a light beam; a first optical system which injects the light beam into the dielectric block so that the light beam is incident on a boundary between the dielectric block and the metal film at a plurality of incident angles which are greater than a critical angle for total reflection; and a light detecting unit which detects a state of attenuated total reflection due to surface plasmon resonance by measuring the intensity of the light beam totally reflected from the boundary. In the sensor, the semiconductor laser unit is driven with a driving current on which a high frequency component is superimposed.
(3) According to the third aspect of the present invention, there is provided a sensor comprising: a dielectric block; a cladding layer formed on a face of the dielectric block; an optical waveguide layer formed on the cladding layer and in contact with a specimen; a semiconductor laser unit as a light source which emits a light beam; a first optical system which injects the light beam into the dielectric block so that the light beam is incident on a boundary between the dielectric block and the cladding layer at a plurality of incident angles which are greater than a critical angle for total reflection; and a light detecting unit which detects a state of attenuated total reflection due to excitation of a propagation mode in the optical waveguide layer, by measuring the intensity of the light beam totally reflected from the boundary. In the sensor, the semiconductor laser unit is driven with a driving current on which a high frequency component is superimposed.
(4) Preferably, the sensor according to each of the first to third aspects of the present invention may also have one or any possible combination of the following additional features (a) to (g).
(a) The semiconductor laser unit may comprise a stabilization unit for stabilizing an oscillation wavelength.
(b) The above stabilization unit may comprise a second optical system which feeds back to the semiconductor laser unit a portion of the light beam emitted from the semiconductor laser unit, and a wavelength selection unit which selects a wavelength of the portion of the light beam.
(c) In the case where the wavelength selection unit is realized by using a bulk grating, the second optical system can be formed as follows.
(i) The second optical system may comprise an optical splitting unit and a reflective grating. The optical splitting unit is arranged in an optical path of the light beam emitted from the light source toward the dielectric block, and splits off a portion of the light beam from the optical path. The reflective grating functions as the wavelength selection unit, and reflects a component of the split-off portion of the light beam having the selected wavelength so that the reflected component of the split-off portion of the light beam retraces the path of the split-off portion of the light beam, and is fed back to the light source.
(ii) The second optical system and the wavelength selection unit may be realized by a partially reflective grating which is arranged in an optical path of the light beam emitted from the light source toward the dielectric block, and partially reflects a portion of the light beam having the selected wavelength so that the partially reflected portion of the light beam is fed back to the light source.
(iii) The second optical system and the wavelength selection unit may be realized by a reflective grating which reflects a portion of backward emission light having the selected wavelength so that the reflected portion of the backward emission light is fed back to the light source, where the backward emission light is emitted from the semiconductor laser unit in the direction opposite to the direction of the light beam incident on the dielectric block.
(d) In the case where the wavelength selection unit is realized by using a narrow-band-pass filter, the second optical system can be formed as follows.
(i) The second optical system may comprise an optical splitting unit and a mirror. The optical splitting unit is arranged in an optical path of the light beam emitted from the light source toward the dielectric block, and splits off a portion of the light beam from the optical path. The mirror reflects the split-off portion of the light beam so that the reflected portion of the light beam retraces the path of the split-off portion of the light beam, and is fed back to the light source. The narrow-band-pass filter as the wavelength selection unit is arranged between the optical splitting unit and the mirror so that only a component of the split-off portion of the light beam having a wavelength selected by the narrow-band-pass filter is fed back to the light source.
(ii) The second optical system may be realized by a half mirror, which is arranged in an optical path of the light beam emitted from the light source toward the dielectric block, and partially reflects the light beam, and feeds back a portion of the light beam to the light source. The narrow-band-pass filter is arranged in the optical path between the light source and the half mirror so that only a portion of the light beam having a wavelength selected by the narrow-band-pass filter is fed back to the light source.
(iii) The second optical system may be realized by a mirror, which reflects a portion of backward emission light, and feeds back the backward emission light to the light source, where the backward emission light is emitted from the semiconductor laser unit in the direction opposite to the direction of the light beam incident on the dielectric block. The narrow-band-pass filter is arranged in the optical path between the light source and the mirror so that only a portion of the backward emission light having a wavelength selected by the narrow-band-pass filter is fed back to the light source.
(e) The wavelength selection unit may be realized by using a fiber grating, which diffracts and reflects a light beam. The fiber grating is an optical fiber having a core in which a plurality of refractive-index varied portions are formed in the core at regular intervals. In this case, the second optical system can be formed as follows.
(i) The second optical system may comprise an optical splitting unit and the fiber grating which realizes the wavelength selection unit. The optical splitting unit is arranged in an optical path of the light beam emitted from the light source toward the dielectric block, and splits off a portion of the light beam from the optical path. The fiber grating diffracts and reflects a component of the split-off portion of the light beam having the selected wavelength so that the reflected component of the split-off portion of the light beam retraces the path of the split-off portion of the light beam, and is fed back to the light source.
(ii) The second optical system and the wavelength selection unit may be realized by a partially reflective fiber grating which is arranged in an optical path of the light beam emitted from the light source toward the dielectric block, and partially reflects a portion of the light beam having the selected wavelength so that the partially reflected portion of the light beam is fed back to the light source.
(iii) The second optical system and the wavelength selection unit may be realized by the fiber grating. In this case, the fiber grating reflects a portion of backward emission light having the selected wavelength so that the reflected portion of the backward emission light is fed back to the light source, where the backward emission light is emitted from the semiconductor laser unit in the direction opposite to the direction of the light beam incident on the dielectric block.
(iv) Note that in the case that the oscillation wavelength is stabilized by optical feedback, it is preferable that the frequency of the high-frequency component superimposed on the semiconductor laser is within the range of 200 MHz-1000 MHz.
(f) It is possible to use as the light source a semiconductor laser unit in which a wavelength stabilization unit is built in, such as a DFB (distributed feedback) laser or DBR (distributed Bragg reflector) laser. In this case, the oscillation wavelength can be stabilized without providing the second optical system for optical feedback.
(g) Alternatively, it is possible to stabilize the oscillation wavelength by electrically and finely controlling the temperature and the driving current of the semiconductor laser unit.
(5) The present invention has the following advantages.
(a) As a result of the inventor""s investigation, the inventor has recognized that mode hopping in the semiconductor laser unit causes the aforementioned sudden variations in the output of the light detection unit which detects the state of the attenuated total reflection, and deterioration of the precision in the measurement in the case where the semiconductor laser unit is used as a light source in a sensor utilizing the attenuated total reflection.
Based on the above recognition, the high-frequency current RF is superimposed on the driving current of the semiconductor laser unit so that the oscillation mode of the semiconductor laser unit becomes multiple modes. When the semiconductor laser unit oscillates in multiple modes, variations in the output of the light detection unit caused by the difference in the oscillation mode are averaged. Therefore, high precision in the measurement can be achieved.
(b) When the oscillation wavelength of the semiconductor laser unit in the sensor utilizing the attenuated total reflection is stabilized by the wavelength stabilization unit, it is possible to prevent production of noise or drift in the output of the light detection unit caused by variations in the oscillation wavelength, and improve precision in measurement.
Further, in the case that the oscillation frequency is stabilized by optical feedback, it became evident that by setting the frequency of the high-frequency component to be superimposed on the semiconductor laser within a range of 200 MHz-1000 MHz, the oscillation wavelength (central wavelength) of the semiconductor was stabilized and maintained at a predetermined value. Therefore, by setting the frequency of the high-frequency component within the range described above, a stable measurement signal can be obtained, and particularly high accuracy in measurement becomes possible.